WordPress.org

Author: Igor Prochazka

"play-button" lets you place one or more simple mp3 play buttons to your posts, pages or widget contents. It's as easy as placing the simple [play-button:mysong.mp3] mark-up anywhere in your text.

This plugin uses the free flash-based mp3 player by Fabricio Zuardi and the "XSPF Web Music Player" team.

More info at this plugin's homepage. See a real-world use example at this music site.